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Toxins are hard to classify, harder to regulate, and even harder to remove from the products we consume.   

Classifying human neurotoxins can be tricky. While laboratory research has identified more than 1,000 chemicals to be animal neurotoxins, the known list for humans is small by comparison. Only 214 chemicals have been classified as human neurotoxins, and only 12 have been identified as impacting fetal and child development.

So why is it that only a fifth of the known animal neurotoxins are identified as posing risks to humans? Even more worrisome, why are only 1% identified as hazardous to infants and fetuses? The reason lies in the ethical and practical considerations of scientific research: You can’t study toxicity on humans by feeding people arsenic and comparing their levels of brain damage with a control group. So no authoritative top 10 list of the deadliest neurotoxins is possible. We’re in the dark.

Still, a new study out of Denmark argues that while human neurotoxins can be clearly identified when people who are exposed to them later become ill, it’s not so easy to show whether or not small amounts of chemical exposure may have impacts on developing fetuses and infants, and what those affects may be. Or as David P. Rall, the former director of the National Institute of Environmental Health Sciences, who is quoted in the study, says: “If thalidomide had caused a ten-point loss of intelligence quotient (IQ) instead of obvious birth defects of the limbs, it would probably still be on the market.”

So, the study points out, we often do not know which chemicals can cause lost IQ points or other developmental problems in children. However, research has identified more developmental neurotoxicants in recent years – doubling the list since 2006 – as scientists do what are called “epidemiological birth cohort studies.”